Induction machines have been known for well over a century. Usually such machines comprise a generally cylindrical central rotor and an outer annular stator, although linear machines are also known. Further, it is usual for the conductor coils or windings, which extend longitudinally of the stator, to be wound into slots provided in a laminated iron stator core in order to enhance the flux produced by the stator windings, i.e., the stator windings pass between laminated iron “teeth” defined by the sides of the slots. However, in machines whose windings are able to produce very high flux densities (say, in excess of 1.5 Tesla at the air gap between the rotor and the stator), the use of iron stator teeth becomes undesirable, due to increased reactance and higher iron losses resulting from magnetic saturation of the stator teeth. Consequently, in such machines the iron teeth are conveniently replaced by non-magnetic teeth for support of the stator windings. The air gap between the periphery of the rotor and the beginning of the iron stator core now effectively extends to the bottom of the stator slots. Because the stator winding is fully within this air gap, this type of construction, to which the present invention particularly relates, is known as an “air gap winding”.
Some form of cooling of the stators of such machines is of course required. In general, cooling of stators of induction machines is a well known problem which has been solved in various ways, e.g., by means of cooling passages extending axially and/or radially through the stator. International Publication No. WO 01/17094 A1, for example, shows radial cooling air passages provided between adjacent stacks of toothed laminations in an iron stator core.
However, such high flux densities as that quoted above enables design of much smaller machines having higher power densities, which results in greater generation of heat within the stator windings, but at the same time a much reduced surface area for cooling. This necessitates a more efficient cooling system than known arrangements can provide, in order to prolong the life of the machine.